1. Field of the Invention
The present invention relates to a carbon electrode for a nonaqueous secondary battery, a preparation process therefor and a nonaqueous battery using the same. More particularly, the invention relates to a carbon electrode for a nonaqueous secondary battery which is highly safe and ensures a high capacity and long lifetime, a preparation process therefor and a nonaqueous secondary battery using the same.
2. Related Art
As portable electronic systems, information systems and the like have been remarkably downsized and lightened, secondary batteries are important components for driving these devices. Among such secondary batteries, a lithium secondary battery has a light weight and high energy density. Therefore, the lithium secondary battery is a promising power source for the driving of a portable system and is now under intensive research and development.
In general, metalic lithium is employed as a negative electrode for the lithium secondary battery. However, a dendrite appears and grows on metalic lithium during repeated charge-discharge cycles, resulting in a short circuit within the secondary battery. This makes it difficult to use metalic lithium for the negative electrode for the secondary battery. To prevent the generation of the dendrite, a proposal has been made that a lithium alloy such as lithium-aluminum alloy is used for the negative electrode instead of metalic lithium. Even with the use of the lithium alloy, repeated charge-discharge cycles or exhaustive charge-discharge process may result in segregation of the alloy, making it difficult to obtain satisfactory charge-discharge cycle characteristics.
To cope with this problem, there have been proposed secondary batteries having negative electrodes which include carbon materials as a host and utilize a lithium-ion intercalation-deintercalation reaction. Such secondary batteries have been researched and developed, and some of them have been put into practical application. The lithium secondary batteries using the carbon materials for the negative electrodes thereof are excellent in the cycle characteristic and safety. However, the carbon materials have a variety of allotropic forms ranging from graphite to amorphous carbon, and the characteristic values thereof vary depending on the allotropic forms. The minute structures (including hexagonal net faces) of the carbon materials are also different depending on the allotropic forms. Various carbon materials have been proposed, as the allotropic forms, characteristic values and minute structures thereof significantly influence the characteristics of the resulting electrodes.
For example, negative electrodes using relatively amorphous carbon materials are disclosed in Japanese Unexamined Patent Publications No. 61-111907 (1986) and No. 62-90863 (1987). Negative electrodes using graphite carbon materials are disclosed in Japanese Unexamined Patent Publications No. 60-221964 (1985), No. 4-155776 (1992) and No. 4-115467 (1992). Negative electrodes using surface-treated graphite carbon materials are disclosed in Japanese unexamined Patent Publications No. 4-368778 (1992), No. 5-114421 (1993) and No. 5-121066 (1993). Negative electrodes using carbon materials having particular minute structures regardless of the crystallinity thereof are disclosed in Japanese Unexamined Patent Publications No. 4-280068 (1992) and No. 4-342958 (1992).
However, these carbon materials are all in the form of powder or fiber, thereby requiring a binder for the preparation of the carbon electrodes. Although the characteristics of the carbon materials are excellent, the electrodes using the carbon materials may not exhibit a desired cycle characteristic in practical applications.
Japanese Unexamined Patent Publications No. 60-36315 (1985) and No. 62-24555 (1987) propose processes for directly depositing a carbon material on a metal substrate to be used as a collector. The preparation of the carbon material is achieved by a vapor phase growth method. The carbon material prepared by the vapor phase growth method has excellent characteristics. Particularly, the carbon material directly deposited on the metal collector requires no binder, and exhibits an excellent conductivity. Therefore, an electrode using the carbon material has a high capacity and exhibits a highly stable cycle characteristic.
In Japanese Unexamined Patent Publications No. 59-188578 (1984) and No. 63-24585 (1988), there are disclosed a negative electrode carrying a catalytic substance for catalyzing the polymerization of a polymeric material and a negative electrode including a carbon material deposited on a catalytic substrate, respectively. In the former case, the polymeric material is used as a battery active material, and the catalytic substance is added to catalyze the polymerization of the polymeric material. Therefore, the function of the catalytic substance is different from that of a catalytic substance used in the present invention. In the latter case, the carbon material is graphitized to a greater extent and deposited tightly on the substrate at a low temperature. As is known to those skilled in the art, the negative electrode has a high capacity and exhibits an excellent cycle characteristic.
However, the carbon electrodes and preparation processes therefor described above suffer from several problems.
The chemical vapor deposition method involves a high cost and a difficulty in controlling the deposition of the carbon material to form a uniform film. Although the crystallinity of the carbon material can be improved by increasing the temperature for the deposition of the carbon material, the increased temperature makes it difficult to obtain a carbon electrode having a greater thickness.
Carbon electrodes which are prepared by impregnating a precursor of a carbon material into a metal collector of three-dimensional structure and then solidifying the carbon material are disclosed in Japanese Unexamined Patent Publications No. 4-92364 (1992) and No. 5-347155 (1993). In accordance with the preparation processes disclosed in these publications, the yield of the carbon material from the precursor is low, thereby making it difficult to obtain a high density carbon electrode. Where the metal collector is highly porous, it is difficult to sufficiently catalyze the precursor for carbonization thereof. Further, the precursor should be heat-treated at a temperature lower than a melting point of a metal used for the metal collector. Therefore, the crystallinity of the carbon material cannot be sufficiently increased, resulting in an unsatisfactory charge-discharge capacity of the resulting electrode.
The carbon material may be heat-treated for graphitization thereof after the preparation of the carbon electrode. However, the heat treatment should be performed at a temperature lower than the melting point of the metal used for the metal collector and, therefore, the carbon material cannot be sufficiently graphitized.
Another known carbon electrode uses a powdery active material such as graphite particles as a carbon material. The preparation of the carbon electrode involves a complicated process, in which a graphite material is prepared and pulverized into particles, then the graphite particles are mixed with an appropriate binder, and the mixture is applied on a collector. The carbon electrode using the graphite particles suffers from an irreversible capacity during an initial charge-discharge cycle, which makes it difficult to obtain a higher capacity battery.
To eliminate the irreversible capacity, there is reported a method in which surfaces of the graphite particles are chemically pretreated for reduction thereof, or the graphite particles are coated with an amorphous carbon material.
Such a method improves the initial charge-discharge efficiency, but involves an increased number of process steps for the preparation of the carbon electrode like the aforesaid case. Further, the carbon electrode includes a binder for binding the graphite particles and, hence, the energy density thereof cannot be satisfactorily increased. In addition, the resulting carbon electrode does not exhibit a satisfactory long-term cycle characteristic. Therefore, fundamental solutions to these problems have not been found yet.